Each programming language major mode customizes the definition of
balanced expressions to suit that language. Balanced expressions
typically include symbols, numbers, and string constants, as well as
any pair of matching delimiters and their contents. Some languages
have obscure forms of expression syntax that nobody has bothered to
implement in Emacs.

By convention, the keys for these commands are all Control-Meta
characters. They usually act on expressions just as the corresponding
Meta characters act on words. For instance, the command C-M-b
moves backward over a balanced expression, just as M-b moves
back over a word.

To move forward over a balanced expression, use C-M-f
(forward-sexp). If the first significant character after point
is an opening delimiter (`(' in Lisp; `(', `[' or
`{' in C), C-M-f moves past the matching closing
delimiter. If the character begins a symbol, string, or number,
C-M-f moves over that.

The command C-M-b (backward-sexp) moves backward over a
balanced expression. The detailed rules are like those above for
C-M-f, but with directions reversed. If there are prefix
characters (single-quote, backquote and comma, in Lisp) preceding the
expression, C-M-b moves back over them as well. The balanced
expression commands move across comments as if they were whitespace,
in most modes.

C-M-f or C-M-b with an argument repeats that operation the
specified number of times; with a negative argument, it moves in the
opposite direction.

Killing a whole balanced expression can be done with C-M-k
(kill-sexp) or C-M-DEL (backward-kill-sexp).
C-M-k kills the characters that C-M-f would move over, and
C-M-DEL kills the characters that C-M-b would move
over. On some machines, C-M-DEL typed on the console is a
command to reboot; when that is so, you cannot use it as an Emacs
command. This conflict is rare, though: usually the DEL key for
Emacs is really BACKSPACE, and the reboot command is
C-M-DELETE, so there is no conflict.

A somewhat random-sounding command which is nevertheless handy is
C-M-t (transpose-sexps), which drags the previous
balanced expression across the next one. An argument serves as a
repeat count, and a negative argument drags the previous balanced
expression backwards across those before it (thus canceling out the
effect of C-M-t with a positive argument). An argument of zero,
rather than doing nothing, transposes the balanced expressions ending
at or after point and the mark.

To set the region around the next balanced expression in the buffer,
use C-M-@ (mark-sexp), which sets mark at the same place
that C-M-f would move to. C-M-@ takes arguments like
C-M-f. In particular, a negative argument is useful for putting
the mark at the beginning of the previous balanced expression.

In languages that use infix operators, such as C, it is not possible
to recognize all balanced expressions as such because there can be
multiple possibilities at a given position. For example, C mode does
not treat `foo + bar' as a single expression, even though it
is one C expression; instead, it recognizes `foo' as one
expression and `bar' as another, with the `+' as punctuation
between them. Both `foo + bar' and `foo' are legitimate
choices for "the expression following point" when point is at the
`f', so the expression commands must perforce choose one or the
other to operate on. Note that `(foo + bar)' is recognized as a
single expression in C mode, because of the parentheses.